On 9/21/23 21:08, Eric Dumazet wrote:
> On Thu, Sep 21, 2023 at 9:03 PM Ilya Maximets <[email protected]> wrote:
>>
>> do_execute_actions() function can be called recursively multiple
>> times while executing actions that require pipeline forking or
>> recirculations. It may also be re-entered multiple times if the packet
>> leaves openvswitch module and re-enters it through a different port.
>>
>> Currently, there is a 256-byte array allocated on stack in this
>> function that is supposed to hold NSH header. Compilers tend to
>> pre-allocate that space right at the beginning of the function:
>>
>> a88: 48 81 ec b0 01 00 00 sub $0x1b0,%rsp
>>
>> NSH is not a very common protocol, but the space is allocated on every
>> recursive call or re-entry multiplying the wasted stack space.
>>
>> Move the stack allocation to push_nsh() function that is only used
>> if NSH actions are actually present. push_nsh() is also a simple
>> function without a possibility for re-entry, so the stack is returned
>> right away.
>>
>> With this change the preallocated space is reduced by 256 B per call:
>>
>> b18: 48 81 ec b0 00 00 00 sub $0xb0,%rsp
>>
>> Signed-off-by: Ilya Maximets <[email protected]>
>> ---
>> net/openvswitch/actions.c | 20 +++++++++-----------
>> 1 file changed, 9 insertions(+), 11 deletions(-)
>>
>> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
>> index 5f8094acd056..80cc5c512d7b 100644
>> --- a/net/openvswitch/actions.c
>> +++ b/net/openvswitch/actions.c
>> @@ -312,10 +312,16 @@ static int push_eth(struct sk_buff *skb, struct
>> sw_flow_key *key,
>> }
>>
>> static int push_nsh(struct sk_buff *skb, struct sw_flow_key *key,
>> - const struct nshhdr *nh)
>> + const struct nlattr *a)
>
> Presumably this function should be inlined. (one caller only)
>
> I would add noinline_for_stack to make sure the compiler will not play
> games with this attempt.
Yeah, good point! I didn't see it being inlined in my testing, but it's
better to be sure. I'll post v2 with a flag.
>
>> {
>> + u8 buffer[NSH_HDR_MAX_LEN];
>> + struct nshhdr *nh = (struct nshhdr *)buffer;
>> int err;
>>
>> + err = nsh_hdr_from_nlattr(a, nh, NSH_HDR_MAX_LEN);
>> + if (err)
>> + return err;
>> +
>> err = nsh_push(skb, nh);
>> if (err)
>> return err;
>> @@ -1439,17 +1445,9 @@ static int do_execute_actions(struct datapath *dp,
>> struct sk_buff *skb,
>> err = pop_eth(skb, key);
>> break;
>>
>> - case OVS_ACTION_ATTR_PUSH_NSH: {
>> - u8 buffer[NSH_HDR_MAX_LEN];
>> - struct nshhdr *nh = (struct nshhdr *)buffer;
>> -
>> - err = nsh_hdr_from_nlattr(nla_data(a), nh,
>> - NSH_HDR_MAX_LEN);
>> - if (unlikely(err))
>> - break;
>> - err = push_nsh(skb, key, nh);
>> + case OVS_ACTION_ATTR_PUSH_NSH:
>> + err = push_nsh(skb, key, nla_data(a));
>> break;
>> - }
>>
>> case OVS_ACTION_ATTR_POP_NSH:
>> err = pop_nsh(skb, key);
>> --
>> 2.41.0
>>
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